Hi Dionysius
As the name would imply, it is ring of polyurethane, usually about 1-2 centimetres wide, usually around the sides and back of the hair system at the edge in a horseshoe shape. Sometimes the polyurethane is a separate material which is stuck onto the lace, and sometimes it is actually fused onto the lace (which is better). Sometimes you see a system where the poly ring goes all the way around, including the hairline (not a good design in my view).
The main advantage of the poly is that it is a smooth tough surface for sticking tape to. The tape gets a better adhesion to the poly than it would to lace, and therefore you get a more secure hold. Then, when the time comes to take the piece off, the poly is easier to clean than lace, because it is tougher. You can just rip the used tape off the poly. If you did that with lace, you would almost certainly pull hair through to the underside of the base, and you might pull some hair out altogether or even rip the mesh of the base. The poly also has the secondary advantage that it gives the system a bit of defined shape, which makes it easier to put on. Pure lace by itself can be a bit floppy.
The disadvantages of the poly are (1) it is slightly more detectable to feel, particularly when the tape is on it - someone digging their hands into your hair could feel the slightly raised bump caused by the poly and tape. Also (2) it stops alcohol getting through to the tape to break the adhesive bond, which makes your system slightly harder to get off.
In summary, the poly is a convenience worth having around the back and sides unless you are going to have people feeling in your hair who don't know you wear a system, in which case you are slightly increasing the odds of your being caught out.
Noah
